This invention relates to a color control system for an image forming apparatus, and more particularly to a color control system for an image forming apparatus which optically scans a document to form an image of the document on an image forming medium.
In the field of image forming apparatuses, there has recently been proposed a thermal color copy machine having a thermal ink ribbon with a plurality of colors for color copying. For copying a document, this machine optically scans the document by a scanner to gather image information of the document in terms of color light signals. The image information is converted into color signals corresponding to the colors of the thermal ink ribbon. According to the color signals converted, corresponding ink sections of the thermal ink ribbon are respectively selected to thermally transfer inks onto an image forming medium such as paper by means of a thermal head. The color transfer to the paper is performed for each color in a successive manner.
For changing copy density as desired, the prior machine changes the amount of light emitted from a light source which illuminates the document, such as a fluorescent lamp, by controlling the phase angle of the power voltage applied to the light source. The phase angle control inevitably generates noise. The noise can possibly give rise to an erroneous operation of the copy machine. This is a serious problem particularly for the color copy machine, because the color copy machine has a more complicated circuit than the monocolor copy machine, and therefore is very sensitive to noise.
Most of the prior color copy machines are very inflexible in changing from one color mode to another, for example, from a two-color mode to a single color mode, or from a single color mode to a full-color mode, etc. In contrast with this, actual color copying requires a fairly flexible color mode selection to obtain the best copy. For example, a full-color mode is preferable for photographs, two to seven color modes are preferable for graphs, and a single color made is preferable for characters.